1. Field of the Invention
The invention relates to a production method of producing a fiber-reinforced resin molding by kneading a resin with reinforcing fibers and then placing or charging a kneaded mixture into a molding device.
2. Description of Related Art
Being lightweight and highly strong, fiber-reinforced resin moldings (fiber-reinforced plastics (FRP)), which are made of resins mixed with reinforcing fibers, are used in various industrial fields, such as the automobile industry, the construction industry, and the aviation industry.
For example, in the automobile industry, the fiber-reinforced resin moldings are applied to vehicular framework structural members, such as pillars, rockers, and lower floors, as well as to vehicular non-structural members, such as hood panels, which are required to present a fine appearance. This makes it possible to reduce the weight of vehicles while ensuring the sufficient strength of the vehicles, thereby contributing to the attempts to produce fuel-efficient and eco-friendly vehicles.
There is a wide variety of production methods of producing fiber-reinforced resin moldings described above. In one of the production methods, a kneader, such as a twin-screw extruder, is used. According to this method, first, resin pellets are fed into a resin melting zone of the kneader, where the resin pellets are melted, and then the molten resin is extruded from the resin melting zone. Then, bundles of reinforcing fibers are fed into a kneading zone of the kneader, where the molten resin is kneaded with the reinforcing fibers to produce a kneaded mixture. Then, the kneaded mixture is placed or charged into a molding device. In this way, a fiber-reinforced resin molding is produced.
Examples of reinforcing fibers to be fed into a kneader include carbon fibers and glass fibers. From the viewpoint of the ease of handling and the production efficiency, a roving, which is a bundle of, for example, 12K (12000), 24K (24000), or 50K (50000) fibers (monofilaments) bound together with a binder, is fed, as it is, into a kneader, cut in the kneader, and then kneaded with molten resin to produce a kneaded mixture.
U.S. Pat. No. 6,776,596 describes a fiber-reinforced resin production apparatus in which a resin (thermoplastic resin) and a roving of reinforcing fibers are fed into a twin-screw extruder, and the roving of reinforcing fibers is kneaded with the thermoplastic resin while the reinforcing fibers are cut due to the rotation of screws of the twin-screw extruder, whereby a kneaded mixture is produced.
As described above, a kneaded mixture for a fiber-reinforced resin molding is produced by feeding bundles of reinforcing fibers into a kneader. In this way, the reinforcing fibers, which are easy to handle, need not be subjected to any process. As a result, the production efficiency improves. However, the reinforcing fibers are bound together into bundles, and thus the external force applied from the kneader to the bundles of fibers significantly varies between an inner portion and an outer portion of each bundle. As a result, the fiber lengths of the reinforcing fibers after being broken in the kneader exhibit a wide distribution (wide range of variations). The fiber lengths are widely distributed (vary within a wide range), for example, from a length of 1 mm or shorter to a length of 25 mm or longer. A fiber-reinforced resin molding made of a kneaded mixture containing reinforcing fibers having a wide length distribution has physical property variations, and thus becomes an inferior-quality molding having strength variations.